import turtle
kame = turtle.Turtle()
kame.shape('turtle')
plus = 1
while True:
    kamenoichi = kame.xcor()
    kamenoichi = kamenoichi + plus
    kame.goto(kamenoichi, 0)
    if kamenoichi > 300:
        plus = plus * -1
    #もし変数kamenoichiが - 300 より小さくなったら以下を実行
    if kamenoichi < - 300:
        plus = plus * -1
turtle.mainloop()